The present patent application is directed to low density paperboard and, more particularly, to low density paperboard having a smooth surface.
Paperboard is commonly used in various packaging applications. For example, aseptic liquid packaging paperboard is used for packaging beverage containers, boxes and the like. Therefore, customers often prefer paperboard having a generally smooth surface with few imperfections to facilitate the printing of high quality text and graphics, thereby increasing the visual appeal of products packaged in paperboard.
Conventionally, paperboard smoothness is achieved by a wet stack calendering process in which the paperboard is rewetted and passed through a calendering device having two or more hard rolls. The wet stack calendering process smoothes the paperboard by compressing the fiber network to eliminate the pits and crevices in the raw stock board. Therefore, smooth paperboard is typically more dense (i.e., less bulky) than less smooth paperboard.
For example, in FIG. 1, the basis weight in pounds per ream (1 ream 3000 ft2) of certain prior art solid bleached sulfate (SBS) paperboard products is plotted against caliper thickness (1 point=0.001 inch), thereby providing a visual representation of prior art paperboard density (i.e., basis weight divided by caliper thickness). As can be seen, the data points generally fall within a range between curve R1 and curve R2. Lower density paperboard (i.e., paperboard falling below curve R1), particularly low density paperboard having a smooth surface, has not been observed in the prior art.
Nonetheless, low density is a desirable quality in many paperboard applications. However, preparing a smooth paperboard using the conventional wet stack calendering process requires substantially increasing the paperboard density.
Accordingly, there is a need for a low density paperboard that provides the desired smoothness for high quality printing, while reducing manufacturing cost.